This invention relates generally to the art of protecting earth-installed metallic structures from corrosion caused by natural, battery-type, reactions between the metallic structures and the earth environment and, more particularly, it relates the anode structures for systems which are used to impress electrical potentials on the metallic structures for preventing such corrosion.
The following terms are defined for purposes of this application:
ANODE--An electrode for an electrical circuit from which electrical current flows into an electrolyte at which oxidation (corrosion) of the surface thereof occurs. Electrodes, including anodes, are never ionized.
CATHODE--An electrode of an electrical circuit into which electrical current flows from an electyolyte at which reduction occurs.
CATHODIC PROTECTION--A technique to reduce corrosion of a metal surface by passing sufficient cathodic current to it to cause its anodic reaction (oxidation and other corrosion) rate to become negligible.
ELECTROLYTE--An ionized chemical substance or mixture containing ions which migrate in an electric field to thereby cause an electrical current to pass therethrough.
It has long been known that earth-installed metallic structures, such as pipelines, oil well structures, and the like, corrode because such structures normally contain both anodic and cathodic areas reacting through air, ground or water electrolytes. In anodic areas, the electric currents flow away from the metallic structures into the surrounding electrolytes and cause oxidation corrosion in such anodic areas. Where electric currents flow from the electrolytes into the metallic structures (cathodic areas), there is negligible corrosion. Thus, in order to prevent such oxidation a corrosion (cathodic) protection system impresses an electrical potential between a ground-embedded electrode and a metallic structure to be protected, such that the electrode is anodic, and the metallic structure is cathodic. By adjusting the amount of current caused by this potential, any corroding currents from anodic areas of the metallic structure are overpowered such that there is a net current flow into the metallic structure at all points. Therefore, the entire surface of the metallic structure will be cathodic and corrosion of the metallic structure is thereby prevented. The ground-embedded electrode (anode), however, corrodes and, in a sense, corrosion is simply being transferred from the metallic structure being protected to the embedded anode.
The efficiency of a ground-embedded anode is a function of such factors as the electrical resistivity of the earth in which it is buried, the depth of burial, the number of anodes, the spacing between anodes, the distance to earth-installed metallic structures to be protected, the efficiency of the anode's connection with the earth, etc. Inasmuch as the resistivity of the soil is usually lower at a greater depth, it is desirable to dispose such anodes as deep as possible. Although some have suggested placing such anodes at depths down to 800 feet, in common practice anodes are normally not embedded much more than 100 feet. One reason for this is that once the anode material has served its intended purpose and has been consumed, the anode-well installation is usually abandoned and a new one is dug. The effective working life of a prior-art anode varies, but rarely exceeds 15 to 20 years. Unfortunately, the metallic systems which these anode installations are meant to protect often have useful lives much longer than this, thereby necessitating the installation of a whole new anode system. Because the first anode system usually must be abandoned and a new one installed, the overall cost of corrosion protection can become prohibitative. It is therefore an object of this invention to provide a cathodic protection anode system which will last as long as the metallic structure which it is intended to protect and for which it is therefore economically practical to dig a deep initial hole for embedding the anode.
There have been a number of suggested anode systems in which the anodes are replaceable and therefore whole new anode systems are not required. Tatum (U.S. Pat. Nos. 4,170,532 and 3,725,669) describe systems wherein anodes located in earth holes are covered with a carbonaceous backfill which provides an electrolyte, or conductor, between the anode and the earth and which is easier to remove for replacing the anode than earth. However, removal of the carbonaceous backfill is still difficult and will only be undertaken when anodes have been essentially completely consumed. In this respect, as anodes are consumed, their electrical properties, such as resistance and conductivity, change due to changes in sizes of the anodes, build-up of reaction (oxidation and other corrosion) material around the outer surfaces of the anodes, and changes in contact between the anodes and the electrolyte in which they are contained. Such changing properties result in ever-decreasing corrosion protection efficiency of most prior-art anode systems. But, in any case, removal of the carbonaceous backfill in the Tatum patents is too difficult to undertake for routine cleaning, or renewal of the anodes. It is an object of this invention to provide anodes for a cathodic protection system the size of which can be continually maintained without the laborious and time-consuming job of removing backfill and the electrical properties of which are not unduly changed due to a build-up of reaction (oxidation and other corrosion) materials, changes in shapes and changes in sizes. In fact, it is an object of this invention to provide an anode for a cathodic protection system whose size and condition can be continually maintained to control the density of electrical current flow in a cathodic protection system while maintaining near perfect electrical contact with the surrounding earth. It is also an object of this invention to provide such an anode which can be installed without the necessity of stocking a special backfill material and which can be continually serviced and renewed without removing it from its operating position.
Schutt (U.S. Pat. No. 4,400,259) describes a buried anode assembly in which the anode is formed of a continuous, rope-like element. The size of this anode is not variable and build up of corrosion products, and changes in size from such corrosion, will still significantly affect contact between this anode and the surrounding earth. Further, this anode must be periodically pulled out of its well and replaced. It is an object of this invention to provide an anode which does not have to be periodically removed from a well, whose performance is not significantly affected by corrosion, which does not have to be replaced and whose size is selectively variable without adversely affecting its contact with the earth.
Peterson et al. (U.S. Pat. No. 4,318,787 and 4,201,637) describe sacrificial anodes for use in the ocean in which compositions can be introduced into anode housings through conduits. In the system described by Peterson et al. (U.S. Pat. No. 4,318,787), an electrode material comprising a major amount of particulate anode material and a minor amount of a fluid carrier material is pumped, or allowed to flow, into an anode extrusion die where it remains in a fixed shape. This anode is not for use in the ground inasmuch as the rigid, paste-like material, would not come into sufficiently intimate contact with the earth and its electrolytes to conduct the required current. Also, in this system one cannot adjust the shape of the electrode in situ inasmuch as the composition is not sufficiently fluid therefor. Still further, because of the looseness of the particulate anode material used for this electrode, an undue amount of anode material is required in order to protect a metallic structure. Yet another difficulty with this anode is that one cannot easily measure the quantity and size of the anode from the surface of the water in which the anode is mounted. This anode also suffers because its electrical characteristics are affected by corrosion of the anode itself; corrosion materials cannot be easily cleaned therefrom in order to renew it. It is an object of this invention, to provide an anode for use in the earth whose size can be selectively maintained and which has a sufficient molecular weight and density that an undue amount thereof is not needed to protect a metallic structure. In addition, it is an object of this invention to provide such an anode whose size can be easily ascertained from the surface of the earth and whose electrical properties are not unduly affected by reaction (oxidation and other corrosion).
It is another object of this invention to provide an anode which can be mounted in an existing, producing, gas or oil well without affecting the production thereof but yet has all of the above-named advantages of this anode.